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This small library was built to support table-per-hierarchy inheritance pattern (TPH) popular in Microsoft's Entity Framework.
TPH uses a single table to store the data for all types in the hierarchy, and a discriminator column is used to identify which type each row represents.

It is similar to Polymorphic Embed with few key differences.
Thanks to this library, those entities can be fully separated structs, which brings many simplifications during inserting and querying.
You can also add any extra fields that will exist only in one struct (for example virtual ones or relationships).


Inheritance, huh?!

It may seem not reasonable to introduce concept of inheritance to the Ecto (and Elixir itself), but for some cases I think it's better to have it instead of repeated code.
Inheritance is natural for our environment, everything comes from some more general being and shares its capabilities.
Without it, you're left just with pattern matching, and it's enough for most cases, but not all of them.

Quick example:
mug and cup, they look almost the same, but cup can have reference to a saucer.
You could preload this reference for both and just remember that for mugs it's always empty, but this generates extra load on the DB (which doesn't know what you know) and forces you to explain for any new person to the project why this will be missing for mug

So yea, this library introduces a concept of inheritance to your code.



The package can be installed by adding ecto_discriminator to your list of dependencies in mix.exs:

def deps do
    {:ecto_discriminator, "~> 0.2.0"}





It has been built to mimic Ecto.Schema as much as possible. That said, the only changes to do in base schema are:

  1. Replace Schema module
defmodule EctoDiscriminator.SomeTable do
-   use Ecto.Schema
+   use EctoDiscriminator.Schema
  1. Define discriminator column
    schema "some_table" do
+   field :type, EctoDiscriminator.DiscriminatorType

You can also mark the primary key as a discriminator

+   @primary_key {:type, EctoDiscriminator.DiscriminatorType, []}
    schema "some_table" do

Then you can add some diverged schemas

defmodule EctoDiscriminator.SomeTable.Foo do
  use EctoDiscriminator.Schema

  schema EctoDiscriminator.SomeTable do
    embeds_one :content, EctoDiscriminator.SomeTable.BarContent

Library will do the rest. Querying for diverged schema automatically adds filter to SQL.



To reduce repetitive usage of cast with a list of common fields for diverged schemas you can call cast_base(params) to automatically apply changeset from base schema.
This function won't be available if base schema doesn't have any changeset/2 function

Some may find it useful to insert diverged schema directly from the base (by specifying discriminator value in changeset params).
This is doable by calling diverged_changeset function on base schema.



Refer to the documentation of modules for some examples.

You can also browse test directory for some example setup.


Known limitations

  • When using keywords for constructing queries: from(s in SomeTable) the where condition won't be automatically applied.
    This is because Ecto handles from macro in a way that skips Ecto.Queryable protocol.
  • It's not possible to obtain correct mapping with something like Repo.all(BaseSchema). You have to execute separate query for each diverged type and then concat results.